inference: unwrap `PartialStruct` typ in `precise_container_type` fast path (#61744)

`PartialStruct.typ` may be a `UnionAll` (parametric `Tuple`/`NamedTuple`
produced via `:new`), in which case the `isa(_, DataType)` gate misses
and we fall through to the generic path, discarding the precise
per-field info in `typ.fields`. Unwrap so the fast path engages.

Found as a minor improvement opportunity during the audit done for
JuliaLang/julia#61743. No current code path is known to reach this with
a UnionAll `PartialStruct`, so the change is defensive (thus no tests
are added).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>